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Figure
100 is a photo of the brick tested at U.W. The glazed surface holds a magnet: clear indication that it once lined an
iron furnace. Figure 101 shows the profile of the brick maker, which does not resemble a native American.
CLICK photos to enlarge |
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LUMINESCENCE ANALYSIS OF FIRED CLAY FROM VIRGINIA –
UW3749 29 April 2019 James K. Feathers Luminescence Dating Laboratory University of Washington Seattle, WA 98195-3412 Email: jimf@uw.edu This
report presents the results of luminescence analysis on a brick samplea from
an apparent smelting site in northern Shenandoah Valley, Frederick County,
Virginia. The samples, submitted by
Adam Arkfield, the owner of the property, is a brick with a black glazed
surface (lab number UW3749).
Luminescence was evaluated using fine-grain procedures. Coarse quartz grains were prepared but no data were obtained from them. Laboratory procedures for the fine grains
are given in the appendix. Dose rate The
dose rate was measured on the sample and associated sediment. Dose rates were mainly determined using
alpha counting and flame photometry.
The beta dose rate calculated from these measurements was compared
with the beta dose rate measured directly by beta counting. These differed significantly, possibly due
to disequilibrium in the U decay chain.
The beta dose rate from beta counting was used for age
calculation. Moisture content of
the brick was estimated as 80 ± 20 % of the saturated value, or 7% for
UW3749. Associated sediment was
estimated to have 15 ± 5% moisture content.
Cosmic dose radiation was calculated as explained in the
appendix. Table 1 gives the
radioactivity data and Table 2 gives the dose rate. Table 1.
Radionuclide concentrations
Table 2. Dose
rates (Gy/ka)*
* Dose rates for fine grains are calculated for OSL.
They will be higher for TL due to higher b-values. Also the beta dose rate is lower than
that given in Table 2 due to moisture correction. Equivalent Dose Equivalent
dose on 1-8µm grains was measured for TL, OSL and IRSL as described in the
appendix. For TL, only a narrow
plateau from 250-290°C could be obtained.
A poor plateau could be indicative of poor firing. There was no
sensitivity change with heating.
Measured TL anomalous fading was insignificant, but was based on only
limited data. OSL/IRSL was
measured on 6 aliquots. Scatter was low with over-dispersion of about
10%. A reliable IRSL signal was not
detected. IRSL stems from feldspars,
which are prone to anomalous fading.
No IRSL suggests the OSL is dominated by quartz. Moreover, the OSL b-value, which is a
measure of the efficiency of alpha radiation in producing luminescence as
compared to beta and gamma radiation, is in the typical range of quartz. It is likely the OSL signal stems mainly
from quartz and does not fade. As a
test of the SAR procedures, a dose recovery test was performed but the
derived dose was under-estimated for UW3749 at one sigma, but close at two
sigma. Equivalent dose values and b-values are given in Table 3. Table 3. Equivalent dose and b-value – fine grains
Ages Age
is given in Table 4. The OSL age is
AD 10 ± 160. It was younger for TL, probably
because of anomalous fading. The ages
for two samples submitted previously ranged from AD 150 to AD 170. Prehistoric smelting has previously been
unknown in the United States, although it was present in South America about
this time. The older ages obtained
here may reflect uneven heating of the samples, where portions of the sample
did not have their luminescence signal completely reset at the time of
firing. Narrow plateaus may also
reflect uneven firing. Table 4. Ages
Procedures for
Thermoluminescence Analysis of Ceramics Sample
preparation -- fine grain The fired clay is
broken to expose a fresh profile. Material
is drilled from the center of the cross-section, more than 2 mm from either
surface, using a tungsten carbide drill tip.
The material retrieved is ground gently by an agate mortar and pestle,
treated with HCl, and then settled in acetone for 2 and 20 minutes to
separate the 1-8 µm fraction. This is
settled onto a maximum of 72 stainless steel discs.. Glow-outs Thermoluminescence
is measured by a Daybreak reader using a 9635Q photomultiplier with a Corning
7-59 blue filter, in N2 atmosphere at 1°C/s to 450°C. A preheat of 240°C with no hold time
precedes each measurement. Artificial
irradiation is given with a 241Am alpha source and a 90Sr
beta source, the latter calibrated against a 137Cs gamma
source. Discs are stored at room
temperature for at least one week after irradiation before glow out. Data are processed by Daybreak TLApplic
software. Fading
test Several discs are
used to test for anomalous fading.
The natural luminescence is first measured by heating to 450°C. The discs are then given an equal alpha
irradiation and stored at room temperature for varied times: 10 min, 2 hours,
1 day, 1 week and 8 weeks. The
irradiations are staggered in time so that all of the second glows are
performed on the same day. The second
glows are normalized by the natural signal and then compared to determine any
loss of signal with time (on a log scale).
If the sample shows fading and the signal versus time values can be
reasonably fit to a logarithmic function, an attempt is made to correct the
age following procedures recommended by Huntley and Lamothe (2001). The fading rate is calculated as the
g-value, which is given in percent per decade, where decade represents a
power of 10. Equivalent
dose The equivalent dose
is determined by a combination additive dose and regeneration (Aitken
1985). Additive dose involves
administering incremental doses to natural material. A growth curve plotting dose against
luminescence can be extrapolated to the dose axis to estimate an equivalent
dose, but for pottery this estimate is usually inaccurate because of errors
in extrapolation due to nonlinearity.
Regeneration involves zeroing natural material by heating to 450°C and
then rebuilding a growth curve with incremental doses. The problem here is sensitivity change
caused by the heating. By
constructing both curves, the regeneration curve can be used to define the
extrapolated area and can be corrected for sensitivity change by comparing it
with the additive dose curve. This
works where the shapes of the curves differ only in scale (i.e., the
sensitivity change is independent of dose).
The curves are combined using the “Australian slide” method in a
program developed by David Huntley of Simon Fraser University (Prescott et
al. 1993). The equivalent dose is
taken as the horizontal distance between the two curves after a scale
adjustment for sensitivity change.
Where the growth curves are not linear, they are fit to quadratic
functions. Dose increments (usually
five) are determined so that the maximum additive dose results in a signal
about three times that of the natural and the maximum regeneration dose about
five times the natural. A plateau region is
determined by calculating the equivalent dose at temperature increments
between 240° and 450°C and determining over which temperature range the
values do not differ significantly.
This plateau region is compared with a similar one constructed for the
b-value (alpha efficiency), and the overlap defines the integrated range for
final analysis. Alpha
effectiveness Alpha efficiency is
determined by comparing additive dose curves using alpha and beta
irradiations. The slide program is
also used in this regard, taking the scale factor (which is the ratio of the
two slopes) as the b-value (Aitken 1985). Radioactivity Radioactivity is
measured by alpha counting in conjunction with atomic emission for 40K. Samples for alpha counting are crushed in
a mill to flour consistency, packed into plexiglass containers with ZnS:Ag
screens, and sealed for one month before counting. The pairs technique is used to separate the U and Th decay
series. For atomic emission measurements, samples are dissolved in HF and
other acids and analyzed by a Jenway flame photometer. K concentrations for each sample are
determined by bracketing between standards of known concentration. Conversion to 40K is by natural
atomic abundance. Radioactivity is
also measured, as a check, by beta counting, using a Risø low level beta GM
multicounter system. About 0.5 g of
crushed sample is placed on each of four plastic sample holders. All are counted for 24 hours. The average is converted to dose rate
following Bøtter-Jensen and Mejdahl (1988) and compared with the beta dose
rate calculated from the alpha counting and flame photometer results. Both the ceramic
and an associated soil sample are measured for radioactivity. Additional soil samples are analyzed where
the environment is complex, and gamma contributions determined by gradients
(after Aitken 1985: appendix H).
Cosmic radiation is determined after Prescott and Hutton (1994). Radioactivity concentrations are
translated into dose rates following Guérin et al. (2011). Moisture
Contents Water absorption
values for the ceramics are determined by comparing the saturated and dried weights. For temperate climates, moisture in the
pottery is taken to be 80 ± 20 percent of total absorption, unless otherwise
indicated by the archaeologist. Again
for temperate climates, soil moisture contents are taken from typical
moisture retention quantities for different textured soils (Brady 1974: 196),
unless otherwise measured. For drier
climates, moisture values are determined in consultation with the
archaeologist. Procedures
for Optically Stimulated or Infrared Stimulated Luminescence of Fine-grained
ceramics. Optically
stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL) on
fine-grain (1-8µm) samples are carried out on single aliquots following
procedures adapted from Banerjee et al. (2001) and Roberts and Wintle (2001. Equivalent dose is determined by the
single-aliquot regenerative dose (SAR) method (Murray and Wintle 2000). The
SAR method measures the natural signal and the signal from a series of
regeneration doses on a single aliquot.
The method uses a small test dose to monitor and correct for
sensitivity changes brought about by preheating, irradiation or light
stimulation. SAR consists of the
following steps: 1) preheat, 2) measurement of natural signal (OSL or IRSL),
L(1), 3) test dose, 4) cut heat, 5) measurement of test dose signal, T(1), 6)
regeneration dose, 7) preheat, 8) measurement of signal from regeneration,
L(2), 9) test dose, 10) cut heat, 11) measurement of test dose signal, T(2),
12) repeat of steps 6 through 11 for various regeneration doses. A growth curve is constructed from the
L(i)/T(i) ratios and the equivalent dose is found by interpolation of
L(1)/T(1). Usually a zero
regeneration dose and a repeated regeneration dose are employed to insure the
procedure is working properly. For
fine-grained ceramics, a preheat of 240°C for 10s, a test dose of 3.1 Gy, and
a cut heat of 200°C are currently being used, although these parameters may
be modified from sample to sample. The
luminescence, L(i) and T(i), is measured
on a Risø TL-DA-15 automated reader by a succession of two
stimulations: first 100 s at 60°C of IRSL (880nm diodes), and then 100s at
125°C of OSL (470nm diodes).
Detection is through 7.5mm of Hoya U340 (ultra-violet) filters. The two stimulations are used to construct
IRSL and OSL growth curves, so that two estimations of equivalent dose are
available. Anomalous fading usually
involves feldspars and only feldspars are sensitive to IRSL stimulation. The rationale for the IRSL stimulation is
to remove most of the feldspar signal, so that the subsequent OSL (post IR
blue) signal is free from anomalous fading.
However, feldspar is also sensitive to blue light (470nm), and it is
possible that IRSL does not remove all the feldspar signal. Some preliminary tests in our laboratory have
suggested that the OSL signal does not suffer from fading, but this may be
sample specific. The procedure is
still undergoing study. A dose recovery
test is performed by first zeroing the sample by exposure to light and then
administering a known dose. The SAR
protocol is then applied to see if the known dose can be obtained. Alpha
efficiency will surely differ among IRSL, OSL and TL on fine-grained
materials. It does differ between
coarse-grained feldspar and quartz (Aitken 1985). Research is currently underway in the laboratory to determine
how much b-value varies according to stimulation method. Results from several samples from
different geographic locations show that OSL b-value is less variable and centers
around 0.5. IRSL b-value is more variable
and is higher than that for OSL. TL
b-value tends to fall between the OSL and IRSL values. We currently are measuring the b-value for
IRSL and OSL by giving an alpha dose to aliquots whose luminescence have been
drained by exposure to light. An
equivalent dose is determined by SAR using beta irradiation, and the
beta/alpha equivalent dose ratio is taken as the b-value. A high OSL b-value is indicative that
feldspars might be contributing to the signal and thus subject to anomalous
fading. Age and
error terms The
age and error for both OSL and TL are calculated by a laboratory constructed
spreadsheet, based on Aitken (1985).
All error terms are reported at 1-sigma. Ka is thousand years before 2018. References Adamiec,
G., and Aitken, M. J., 1998, Dose rate conversion factors: update. Ancient
TL 16:37-50. Aitken, M. J., 1985, Thermoluminescence Dating, Academic Press, London. Auclair,
M., et al., 2003. Measurement of
anomalous fading for feldspar IRSL using SAR. Radiation Measurements, 37: 487-492. Banerjee,
D., Murray, A. S., Bøtter-Jensen, L., and Lang, A., 2001, Equivalent dose
estimation using a single aliquot of polymineral fine grains. Radiation
Measurements 33:73-93. Bøtter-Jensen,
L, and Mejdahl, V., 1988, Assessment of beta dose-rate using a GM
multi-counter system. Nuclear Tracks and Radiation Measurements
14:187-191. Brady,
N. C., 1974, The Nature and Properties of Soils,
Macmillan, New York. Galbraith,
R. F., and Roberts, R. G., 2012.
Statistical aspects of equivalent dose and error calculation and
display in OSL dating: an overview and some recommendations. Quaternary
Geochronology 11:1-27. Guérin,
G., Mercier, N., and Adamiec, G., 2011, Dose-rate converstion factors:
update. Ancient TL 29:5-8. Huntley,
D. J., and Lamothe, M., 2001, Ubiquity of anomalous fading in K-feldspars,
and measurement and correction for it in optical dating. Canadian
Journal of Earth Sciences 38:1093-1106. Mejdahl,
V., 1983, Feldspar inclusion dating of ceramics and burnt stones. PACT
9:351-364. Murray,
A. S., and Wintle, A. G., 2000, Luminescence dating of quartz using an
improved single-aliquot regenerative-dose protocol. Radiation Measurements
32:57-73. Prescott,
J. R., Huntley, D. J., and Hutton, J. T., 1993, Estimation of equivalent dose
in thermoluminescence dating – the Australian
slide method. Ancient TL 11:1-5. Prescott,
J. R., and Hutton, J. T., 1994, Cosmic ray contributions to dose rates for luminescence
and ESR dating: large depths and long time durations. Radiation
Measurements 23:497-500. Roberts,
H. M., and Wintle, A. G., 2001, Equivalent dose determinations for
polymineralic fine-grains using the SAR protocol: application to a Holocene sequence
of the Chinese Loess Plateau. Quaternary Science Reviews 20:859-863. |
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